SNS-Ligands for Ru-Catalyzed Homogeneous Hydrogenation and

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SNS-Ligands for Ru-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions Johannes Schörgenhumer, Axel Zimmermann, and Mario Waser Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00142 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Organic Process Research & Development

SNS-Ligands for Ru-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions Johannes Schörgenhumer,a Axel Zimmermann,b* and Mario Wasera*

a)

Institute of Organic Chemistry, Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria [email protected]; Tel: +4373224685411; Fax: +437322468545402.

b)

Patheon Austria, part of Thermo Fisher Scientific, St. Peterstr. 25, 4020 Linz, Austria [email protected]

ACS Paragon Plus Environment

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Graphical Abstract


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KEYWORDS: Homogeneous Catalysis; Pincer Type Ligands; Hydrogenation; Acceptorless Dehydrogenation; Carboxylic Acid Derivatives.

ABSTRACT: A detailed study of literature known and novel S-containing pincer type ligands for Ruthenium-catalyzed homogeneous hydrogenation and dehydrogenation reactions was carried out. The scope and limitations of these catalysts were carefully investigated and it was shown that simple bench stable SNS-Ru complexes can be used to facilitate the hydrogenation of a variety of different substrates at a maximum H2 pressure of 20 bar under operationally simple and easy to scale-up glovebox-free conditions by using starting materials and reagents that do not require any special purification prior to use. It was also shown that such complexes can be used to catalyze the dehydrogenative coupling of alcohols and amines to get amides as well as for the dehydrogenative dimerization of alcohols to esters.


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Introduction The homogeneous hydrogenation of carboxylic acid derivatives, i.e. esters, is a transformation of uttermost importance.1 Accordingly, the development of catalyst systems to carry out these reactions under mild and environmentally benign conditions is at the forefront of catalysis research, and a variety of different approaches relying on different metals and ligands have been introduced over the last decade.1-8 Tridentate pincer-type ligands have emerged as one of the most important general structural platforms for homogenous transition metal catalysis.2 Pioneering work by Milstein3 and others,4 who introduced highly active Ru-based PNN or PNP pincer systems inspired the development of a variety of Ru-based catalysts and lead to several impressive applications for homogenous hydrogenation reactions of carboxylic acid derivatives.5 In addition, the last years have witnessed an increasing interest in the replacement of noble metals by cheaper more abundant base transition metals like e.g. Mn or Fe. These catalyst systems can also be used for robust hydrogenations of esters and other functional groups, albeit they usually require higher catalyst loadings than the noble metal-based systems.6 With respect to potential larger scale industrial (ester) hydrogenations the use of highly active homogeneous Ru catalysts represents an attractive opportunity. Hereby some of the main requirements are: Low catalyst loadings; cheap ligands; operationally simple, scalable, and robust conditions avoiding glove-box procedures; H2-pressures below 20 bar that allow for hydrogenations in standard multipurpose batch vessels. One catalyst system that fulfils a lot of these requirements and which has proven its potential on multiton scale is Takasago’s Ru-MACHO complex I, which can even be used for the atmospheric pressure hydrogenation of esters when an additional N-heterocyclic carbene ligand is used (albeit requiring higher loadings).7


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Scheme 1. Targeted investigations of Ru-SNS systems for hydrogenations and dehydrogenations.

A different class of Ru-complexes that has shown very promising catalytic potential in the initial investigations is Gusev’s SNS-based catalyst IIa.8 This catalyst has shown very good activity for ester hydrogenations at 50 bar and we were surprised to see that this complex has not been more frequently used since that impressive first report. A literature research revealed that in general SNS or SNN pincer-type ligands have so far not received that much attention, which came as a surprise to us, considering the simple synthesis of some of these ligands.8,9.10 We therefore became interested in testing this platform for its potential to meet the above mentioned criteria


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(i.e. lower hydrogenation pressure and robust and operationally simple “glove-box free” conditions that may allow for an industrial use) and carried out a broad and systematic testing of different catalyst derivatives and conditions for homogeneous hydrogenations (i.e. of esters, Scheme 1). In addition, Gusev’s and Yang’s groups also carried out detailed mechanistic studies for this catalyst8,11 and Gusev also described the potential of their catalyst for the oxidative dimerization of EtOH to EtOAc.8,11a,b Surprisingly, to the best of our knowledge this is the only thoroughly investigated example for an acceptorless dehydrogenation reaction with SNScontaining pincer-type ligands so far.12 We therefore also carried out a series of different dehydrogenation experiments within this study (Scheme 1, lower reactions). Altogether, we now wish to provide a detailed overview of the potential, robustness, and limitations of different sulfur-based pincer-type ligand-containing Ruthenium complexes for different homogeneous hydrogenation and dehydrogenation reactions that may also hold promise for eventually larger scale applications.

Results and Discussion Ligands synthesized and tested: We started our investigations by preparing13 a variety of easily accessible SNS complexes II in analogy to Gusev’s work (Scheme 2).8 In addition to these simple aliphatic ligand systems we also decided to investigate pyridine-based SNS complexes III14 as well as the analogous SCS complexes IV.15 Besides these symmetric


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systems, we also synthesized a variety of dissymmetric S-containing pincer ligands to access the pyridine-based complexes V-VIII within this study.16

Scheme 2. Ru-complex with S-containing pincer-type ligands tested in this study.

Homogeneous Hydrogenation: To elucidate the catalytic potential of the S-containing catalyst systems shown in Scheme 2 for homogenous hydrogenation reactions under a maximum H2 pressure of 20 bar, we first tested them for the hydrogenation of methyl benzoate 1a under a variety of different conditions (Table 1). In all these reactions we used dry degassed toluene as the solvent (unless otherwise stated) and the catalysts and reagents were weighed, handled, and charged avoiding any glovebox conditions. First experiments were carried out using the parent Gusev system IIa at 10 bar


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(entries 1-9). When using 0.1 mol% of the catalyst we observed almost 40% conversion after 5 h at 60 °C and the reaction rate could significantly be improved by raising the reaction temperature to 100 °C (entries 1-3). Unfortunately, when lowering the catalyst loading stepwise to 0.02 mol% under otherwise identical conditions we observed a dramatic decrease in the reaction rates (entries 4 and 5). Reactions carried out at different concentrations then revealed a pronounced sensitivity of this system. While neat reactions performed well (entry 6), we were not able to identify reproducible and robust conditions under higher dilutions (0.1M). Hereby the conversions varied between almost zero up to 80-90% from experiment to experiment under identical conditions (0.1M concentration of 1a, 10 bar H2, 0.1 mol% IIa, 10 mol% t-BuOK, 80100 °C). We carried out a broad variety of experiments (different qualities of solvents, base, …) to understand the reasons for these seemingly random outcomes, but in the end we were only able to achieve good and reliable conversions under these conditions when using a larger amount of base and longer reaction times (entry 7).


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Table 1. Catalyst screening and identification of robust conditions for the hydrogenation of

methylbenzoate 1a

Entrya

Cat. (mol%)

Base (mol%)

T [°C]

H2 [bar]

t [h]

Conv. [%]b)

1

IIa (0.1%)

t-BuOK (10%)

60

10

5

38

2

IIa (0.1%)

t-BuOK (10%)

80

10

5

78

3

IIa (0.1%)

t-BuOK (10%)

100

10

5

97

4

IIa (0.05%)

t-BuOK (10%)

100

10

5

54

5

IIa (0.02%)

t-BuOK (10%)

100

10

5

12

6c)

IIa (0.1%)

t-BuOK (10%)

100

10

5

93

7d)

IIa (0.5%)

t-BuOK (50%)

80

10

20

95

8e)

IIa (0.1%)

t-BuOK (10%)

80

10

5

19

9f)

IIa (0.1%)

t-BuOK (10%)

80

10

5

36

)


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10

IIa (0.1%)

t-BuOK (10%)

80

20

5

97

11

IIa (0.05%)

t-BuOK (10%)

80

20

5

65

12

IIa (0.1%)

t-BuOK (10%)

80

20

1

75

13

IIa (0.1%)

NaOMe (10%)

80

20

5

50

14

IIa (0.1%)

NaH (10%)

80

20

5

88

15

IIa (0.1%)

KOH (10%)

80

20

5

63

16

IIa (0.1%)

Cs2CO3 (10%)

80

20

5

78

17

IIb (0.1%)

t-BuOK (10%)

80

20

5

100 (96)g)

18h)

IIb (0.1%)

t-BuOK (10%)

80

20

5

78

19i)

IIb (0.1%)

t-BuOK (10%)

100

20

5

89

20

IIc (0.1%)

t-BuOK (10%)

80

20

5

61

21

IId (0.1%)

t-BuOK (10%)

80

20

5

19

22

IIIa (0.1%)

t-BuOK (10%)

80

20

5

3

23

IIIb (0.1%)

t-BuOK (10%)

80

20

5

SNS-Ligands for Ru-Catalyzed Homogeneous Hydrogenation and

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